The invention relates to a method for discrete-time reactance measurement in which the reactance is activated by a clocked generator device with an electric voltage an electric current or an electric charge, and a clocked analyzer device generates an output signal as a function of the reactance to be measured from discrete-time sampled values of the electric voltage, electric current or electric charge, the generator device and analyzer device being clocked in synchronism.
Capacitive and inductive sensors have found wide acceptance for measuring a plurality of physical variables. As a rule, such sensors cannot be analyzed statically, this being the reason why they need to be activated by a generator with an alternating voltage or alternating current. Various possibilities exist for determining the sensor reactance. Due to its good integratability, use is often made of the switched-capacitor technique in which each analysis cycle consists of one or more precharge phases (activation) and one or more measuring phases (analyzing), the switched-capacitor technique thus representing a discrete-time method.
Measuring capacitances by the switched-capacitor technique is known for example from the paper "A Switched-Capacitor Digital Capacitance Meter" by Hiroki Matsumoto and Kenzo Watanabe in the journal "IEEE Transactions on Instrumentation and Measurement", vol. IM-35, No. 4, December 1986, pages 355-359, and from German Patent 35 44 187 A1. In this known method a constant clock is used for activation and analysis so that the time-spacings between the points in times of analysis are always the same. In keeping with the Nyquist sampling theorem it is known that the repetition frequency of the analysis must be at least twice as high as the highest frequency of the variable to be measured, the ratio of the analysis clock frequency to twice the value of the highest frequency of the variable to be measured being termed the oversampling rate. When this ratio is smaller than unity the Nyquist sampling theorem is violated and aliasing errors result.
In the majority of applications the changes in time of the reactance to be measured are relatively slow so that a high oversampling rate is possible as is also often made use of to improve the accuracy of measurement. However, due to external influencing factors, disturbing alternating signals are also coupled into the reactance to be measured. The frequencies of these disturbances may be in ranges which result in violation of the Nyquist sampling theorem. Aliasing then results in the interference signal being imaged on the wanted signal and thus in the reactance measurement being falsified. More particularly, it is in the more immediate vicinity of the whole number multiples of the analysis clock frequency (sampling frequency) that ranges highly sensitive to interference materialize.
On the other hand it is known to vary a system clock by frequency or phase modulation to reduce disturbing effects. Various known solutions in this respect read from EP 0 715 408 A1, and a further solution is stated in which an original clock is modulated by means of a phase modulator as a function of a random signal source. Described in DE 196 37 942 A1 is a method of modulating the detection clock of a digital oscilloscope in making use of a phase-locked loop.